Hybrid and electric vehicles (HEVs) typically include an alternating current (AC) electric motor which is driven by a direct current (DC) power source, such as a storage battery. Stator windings of the AC electric motor can be coupled to a power inverter module that performs a rapid switching function to convert the DC power to AC power to drive the AC electric motor, which in turn drives a shaft of HEV's drivetrain. The temperature of motor stator windings is an important parameter since it can be used for a variety of purposes. For example, stator winding temperature can be an important parameter in various motor control algorithms that utilize stator resistance as a control variable because stator winding resistance is temperature dependent and can be adjusted based on temperature.
Stator winding temperature can also be used to detect high motor temperatures to prevent overheating. Typically, the temperature of the stator windings is measured by a temperature measurement sensor, such as a thermistor or thermocouple that is installed or mounted on one of the electric motor's stator windings. If the three phase currents that flow in the stator windings are balanced, a single temperature measurement sensor can sometimes be used adequately to estimate the temperature of all of the stator windings. However, in some systems, there may be a very large temperature gradient between the temperature sensor and the hot spot of the stator winding. In this situation, using the single temperature sensor to predict the motor hot spot temperature becomes difficult. Additionally, at zero speed, no current may be flowing in one of the stator windings where the sensor is installed or, at certain speeds, unbalanced currents may be flowing in one of the stator windings. For example, during a stall condition, one phase may carry a current equivalent to the peak of the sine wave current, while the other two phases carry one half the current with opposite sign. Hence, one phase may experience four times (4×) the resistive heating losses compared to the other two phases. Under these conditions, the single temperature measurement sensor will not correctly generate the actual temperature of the electric motor and, consequently, the electric motor can be damaged by overheating.
Another drawback is that such temperature sensors can be expensive, unreliable and can require maintenance or servicing. Each sensor adds extra cost to the system, and in some cases it is necessary to employ multiple sensors in the motor to identify the hottest spot of the stator windings. In addition, the sensors require external electrical signal conditioning circuitry to process the sensor signal(s), which further increases cost of the system and potentially reduces system reliability even further. In addition, they need to be serviced and maintained to ensure that they are operating as intended. Moreover, when sensors fail they must be repaired or replaced which can be a challenge since they are usually located inside the motor, for example, in the middle of a stator slot.
To reduce the number of temperature sensors or even completely eliminate the need for sensors, sensorless stator winding temperature estimation techniques have also been developed. Some sensorless stator winding temperature estimation techniques employ complex motor thermal models computed based on machine geometry and its thermal and electrical properties. While these techniques can provide accurate and robust temperature estimation, they require development of a complex motor thermal model. In many cases, information regarding the motor geometry and/or its thermal or electrical properties may not be readily available.
In addition, a high-frequency carrier signal injection technique has also been used for stator temperature estimation; however, this technique assumes that the stator and rotor temperatures are identical, which is not always the case. As such, the accuracy declines as the stator and rotor temperatures drift apart.
Other sensorless stator winding temperature estimation techniques have also been developed that work well for zero or low speed temperature estimation (e.g., below 75 rpm); however, these techniques do not yield accurate estimation results at higher motor speeds.
Accordingly, it is desirable to provide a method, system and apparatus for estimating stator winding temperature over the entire motor operating speed range (i.e., low operating speeds and high operating speeds). It would also be desirable to completely eliminate the need for any stator winding temperature sensors. In addition, it is desirable to provide a method, system and apparatus for estimating stator winding temperature that works at all motor operating speeds (i.e., rotor angular velocities) without using a temperature sensor (e.g., thermistor) coupled to one or more of the stator windings. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.